1. Field of the Invention
The present invention relates to a plastic optical fiber having a three-layered structure comprising a core, a cladding layer, and a protective layer.
2. Description of the Prior Art
Improvements in the light transmission properties of optical fibers have led to their increased use in the fields of optical communication and the like. Conventional optical fibers which have been developed hitherto include those having a core consisting of a glass and those having a core consisting of a plastic material. Conventional glass core optical fibers exhibit excellent light transmission properties, and, therefore, are useful in the field of long-distance communication. However, this type of optical fiber exhibits poor flexibility and, therefore, easily breaks. Accordingly, these optical fibers cannot be advantageously used in the field of short-distance communication, wherein a high-order fabrication is necessary. Plastic core optical fibers, on the other hand, can transmit light only a relatively short distance. However, this type of optical fiber exhibits excellent flexibility and, thus, is very useful in the field of short-distance communication.
Conventional plastic core optical fibers which have been developed hitherto are mainly of a step-index type. A step-index type optical fiber is composed of a core comprising a polymer of a high refractive index and a cladding layer comprising a polymer of a low refractive index. When light is applied to the step-index type optical fiber, the light is transmitted through the core while being reflected entirely on the interface between the core and the cladding layer. Therefore, the important requisites for the optical fiber are that the cladding layer be firmly bonded to the core and that the cladding layer be highly resistant to the formation of cracks therein.
The core constituent is preferably composed of an amorphous material having a high optical transparency. Generally, polymethyl methacrylate or polystyrene is used as the core material.
An optical fiber comprising a core consisting of polystyrene and a cladding layer consisting of polymethyl methacrylate is already known in Japanese Examined Patent Publication (Kokoku) No. 43-8978. The bonding property of polystyrene to polymethyl methacrylate, however, is unsatisfactory. Therefore, the resultant optical fiber has unsatisfactory optical properties at the interface between the core and the cladding layer. On the other hand, there are known several references concerning an optical plastic fiber comprising a core consisting of polymethyl methacrylate exhibiting a high transparency and a cladding layer consisting of a fluorine-containing polymer.
For example, Japanese Examined Patent Publication (Kokoku) Nos. 43-8978, 56-8321, 56-8322, and 56-8323 disclose plastic optical fibers comprising a core consisting of polymethyl methacrylate and a cladding layer consisting of a polymer of fluorinated alkyl methacrylate.
However, these cladding layers which have been disclosed hitherto generally are unsatisfactory in respect to the bonding property to the core constituent, moldability suitable for ensuing a uniform and smooth core-cladding layer interface structure, mechanical resistance to friction and bending, heat resistance capable of withstanding the working environment and processing conditions, resistance to chemicals, and the like. On the contrary, a polymer for a cladding constituent capable of completely meeting these requirements is not known.
Further, a mechanically and chemically protected cable comprising an optical fiber coated with a polymethyl methacrylate resin layer has been proposed in, for example, Japanese Unexamined Utility Model Publication (Kokai) No. 50-154741. However, the light transmission properties of this cable are not concretely described.
The present inventors made extension studies to improve a conventional composite spinning system so as to enhance light reflectance on the interface between. the core and the cladding layer of the resultant optical fiber, thereby enhancing the light transmission properties thereof, and at the same time to eliminate or improve various defects concerning the mechanical properties, heat resistance, and chemical resistance of conventional cladding polymers and to reduce substantially the amount of an expensive cladding polymer to be used, thereby realizing a reduction in the cost of the optical fiber. As a result, the present invention was attained.